Why is this value doing this?

I have a number value that I subtract a given number from when a mouse is clicked and just automatically. The problem is, though, that occasionally I subtract a number more than once very close in time to each other in two different scripts. The value, instead of combining both numbers that are being subtracted, only subtracts one of the numbers. Why is this happening?

Script #1

local value = game.Workspace.NumberValue.Value -- let's assume value is 30
local randomNumber = 5
 
function Down(mouse) -- assuming the mouse is clicked very close to the time before randomNumberTwo is subtracted from the value in script #2 and very close to the time  after randomNumberTwo is subtracted
    value = value - randomNumber
    print(value)
end
 
function Selected(mouse)
    mouse.Button1Down:connect(function() Down(mouse) end
end
 
-- Output = 25

Script #2

local randomNumberTwo = 10
local value = game.Workspace.NumberValue.Value -- since only 1 mouse click has occured, the value should now be 25
 
wait(3)
value = value - randomNumberTwo
print(value)
 
-- Output = 20 | the output should be 15, but it's not

This is a great question! It highlights one of the subtleties and pitfalls of working with any sort of Value Instance.

As you probably understand, the ROBLOX game hierarchy is composed of Services (like Players, Workspace, Lighting, and ReplicatedStorage) and other Instances (like Part, Humanoid, and StringValue).

Inside a Script, you can have a variable that refers to an Instance or you can have a variable that refers to a certain value (a value like 5, "hello", {'a','b','c'}, or -2.7843).

Let us analyse the first line of Script #1

local value = game.Workspace.NumberValue.Value

The above is like saying "store the numerical value of 'NumberValue' in the variable called 'value'.

Now consider this:

local numberValue = game.Workspace.NumberValue

The above is like saying "store a reference to the Instance called "NumberValue" in the workspace.

So, the problem is that at the beginning of your scripts, you are taking just the numerical value from the Instance in the workspace, rather than a reference to that Instance. This means that the value you get at the end will only be a manipulation of the value that the "NumberValue" starts with.

This seems like a good explanation: Both scripts start with 30 as their value, the first one takes 5, leaving 25. The second script takes 10 (from 30), leaving 20.

Here are your scripts re-written to accommodate this slight change: #1

local numberValue = game.Workspace.NumberValue
local randomNumber = 5

function Down(mouse) 
    numberValue.Value = numberValue.Value - randomNumber
    print(numberValue.Value)
end

function Selected(mouse)
    mouse.Button1Down:connect(function() Down(mouse) end)
end

#2

local randomNumberTwo = 10
local numberValue = game.Workspace.NumberValue

wait(3)
numberValue.Value = numberValue.Value - randomNumberTwo
print(numberValue.Value)